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<!DOCTYPE concept
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<concept id="GUID-E2641957-8163-5EF4-B282-FC3FD9CA75A6" xml:lang="en"><title>Sound
Driver Technology</title><shortdesc>Describes the technology concepts that are used in the Sound Driver. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<section id="GUID-CBC17614-34B3-52A1-9C74-F7093A175398"><title>Audio hardware
device</title> <p>An audio hardware device is an individual hardware codec
device together with any associated controller hardware that allows the CPU
to communicate with it. </p> <p>A basic audio hardware device typically provides
two communication paths: an input path for the audio recording and an output
path for audio playback. </p> <p>Most basic audio hardware devices support
full duplex data transfer although some are only half-duplex or maybe just
simplex. Each input or output path may be used to transfer mono data or stereo
data. In the case of stereo this consists of two audio channels, the left
and the right; mono data consists of just a single audio channel. </p> <p>A
more complex audio hardware device could be an AC 97 codec or similar, plus
its associated controller, which can support multiple input and output paths.
Each input or output path may be used to transfer mono data, stereo data or
'multichannel data'. For example, left, right, centre, left and right surround
and Low-Frequency Effects (LFE). </p> </section>
<section id="GUID-7C8FF32F-2E84-57B1-8962-9EDB4D51FEE6"><title>Unit</title> <p>Units
are used to provide access to the various audio hardware devices. Each unit
supports just one communication path this is either input or output. </p> <p>Clients
of the audio hardware system can open a separate connection to each unit.
The mapping between the units on a given phone and the audio hardware devices
themselves is platform specific; this is determined by the implementer of
the Sound Driver PDD for that platform. </p> <p>A basic full-duplex audio
hardware device is presented as two units, one input/record unit and one output/playback
unit. A more complex audio hardware device such as an AC 97 codec may be represented
to the rest of the OS as a number of audio input and output units. </p> </section>
<section id="GUID-74B14664-8D9D-57D2-B090-709136FEB5E3"><title>Audio channel</title> <p>An
audio channel is a data stream between a client and an audio unit. There are
one or more audio channels per driver channel. </p> </section>
<section id="GUID-4D6A01FE-47E7-5A6F-9DFE-F612B880165B"><title>Driver channel</title> <p>A
driver channel is a session between a client and an audio unit. A client may
have driver channels open on more than one unit. </p> <p><note> The difference
between a driver channel and an audio channel. A driver channel is a session
between the client and an audio device which can consist of one or more audio
channels. An audio channel refers to the audio stream, for example, left or
right output.</note> </p> <fig id="GUID-C3B51755-EDB0-5518-83D1-886E9148D65A">
<title>              The relationship between audio channels and device channels.
           </title>
<image href="GUID-2D98DB88-BA48-5EF8-A5F9-0CB8688B0B63_d0e398115_href.png" placement="inline"/>
</fig> </section>
<section id="GUID-5C66667B-55C0-521D-86E3-67593DB381C9"><title>Mono to stereo
conversion</title> <p>Many codecs that support stereo playback can only accept
audio data that is delivered with the samples for each of the channels interleaved,
for example, LRLRLR. For these audio hardware devices, in order to operate
the channel in mono mode and to play audio data which contains only samples
for a single channel it is necessary to perform mono-to-stereo conversion
on the audio data before delivering it to the codec. So, for a section of
mono audio data that contains three samples, lets call them S1, S2 and S3,
each sample is duplicated, so we have S1,S1,S2,S2,S3,S3, with identical samples
being delivered to each channel. </p> <p>Unfortunately, the only way for the
PDD to implement this conversion is for it to allocate a conversion buffer
and to copy each sample twice into this buffer. The PDD has to allocate a
separate conversion buffer for each simultaneous transfer operation it supports,
for example, for the template playback driver, a conversion buffer count equal
to <xref href="GUID-EE69DF4F-3341-355A-837A-5DA1C8A20A67.dita"><apiname>KTemplateMaxTxDmaRequests</apiname></xref>. </p> <p>When performing conversion
in this manner, the maximum transfer size that the PDD can accept from the
LDD becomes half the size of each conversion buffer, and when configured in
this mode, the value returned to the LDD in response to <xref href="GUID-61CC68CB-A01D-3CA0-93D9-F3717ABD6424.dita#GUID-61CC68CB-A01D-3CA0-93D9-F3717ABD6424/GUID-A47005E5-36B4-3795-A4C0-CB67B43802B5"><apiname>DSoundScPdd::MaxTransferLen</apiname></xref> must
equal this value. </p> <p>Likewise, the record data may be delivered by the
audio hardware device only in stereo format with the data samples for each
channel interleaved. If the driver channel is configured in mono record mode,
stereo to mono conversion has to be performed in the PDD to discard each alternate
sample. </p> </section>
</conbody></concept>